A 20-year puzzle as to how the heart regulates contraction appears to have been solved by researchers from the University of Bristol. The findings, published in the journal Biophysics, paves the way to improving our understanding of what goes wrong when the heart fails. When the heart beats (contracts), the contractile machinery is switched on by an increase in calcium within the cell. This increase is produced by a release from intracellular stores activated by a small influx of calcium into the cell during the cardiac electrical signal — known as the action potential.